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Experimental Perspective on Sedimentation fromPlumesM.BursikDepartment of Geology,University at Bualo,SUNYBualo,NY 14260mib@buffalo.eduIAVCEI Working Group on Modeling Volcanic Tephra-Fall HazardsFirst Meeting,14 and 20 July 2003Cities on Volcanoes 3Full ScreenJuly 21,2003Univ.BualoIntroductionUniv.Bualo 1IntroductionThe conservation of mass ux of particles for multiple grain size fractions,Mi,as afunction of time,t,is given by:dMidt= wsMih(1)where wsis the settling speed of a particle in a given size class.Univ.Bualo 1IntroductionThe conservation of mass ux of particles for multiple grain size fractions,Mi,as afunction of time,t,is given by:dMidt= wsMih(1)where wsis the settling speed of a particle in a given size class.This was developed for systems with a oor (Hazen,1904)Univ.Bualo 1IntroductionThe conservation of mass ux of particles for multiple grain size fractions,Mi,as afunction of time,t,is given by:dMidt= wsMih(1)where wsis the settling speed of a particle in a given size class.This was developed for systems with a oor (Hazen,1904)Used extensively in civil engineering for design of settling ponds in wastewatertreatment Univ.Bualo 1IntroductionThe conservation of mass ux of particles for multiple grain size fractions,Mi,as afunction of time,t,is given by:dMidt= wsMih(1)where wsis the settling speed of a particle in a given size class.This was developed for systems with a oor (Hazen,1904)Used extensively in civil engineering for design of settling ponds in wastewatertreatmentWorks for volcanic plumes and umbrella clouds (Bursik et al.,1992)Univ.Bualo 1Advection-DiusionHazen's equation can be derived from the advection-diusion equationAssume that there is no relative diusion@tCi+r ~uCi= r2Ci+ (2)Univ.Bualo 2Layered EnvironmentNeeded to test Hazen's validity for layered systemsSet up two main types of experimental systemsIn the layered environment,a light,particle-bearing uid lies atop a denser uidwith no particleUniv.Bualo 3Univ.Bualo 4Univ.Bualo 4Settling PhenomenologyIn experiments on layered systems,we found that...Particles do not always settle from the upper layer singlyThere are particle-laden,descending plumesUniv.Bualo 5Settling PhenomenologyUniv.Bualo 5TheoryThe development of descending plumes has been characterized by Hoyal et al.(1999a,b):Convective sedimentation:Gr =gC0312; = wst (3)Double diusion:Gr =gC021ws(4)Criterion:Gr > 1000 (5)(6)Univ.Bualo 6Shear and Re-entrainmentShear at the boundary between layers can destroy the convection,if the turbulencelevel in the upper layer is suciently vigorous (Gupta,2002)The result is...Univ.Bualo 7Shear and Re-entrainmentUniv.Bualo 7TheoryBecause of in ow toward the plume caused by wind and atmospheric entrainment,particles can be re-entrained.In general,Eqn1takes on a form:dMi dt= f (wsw)Mih(7)where wis a re-entrainment speed.The function,f,can be determined empirically.Univ.Bualo 8Lee Wave,Separation,Coanda Eect...For plumes in a highspeed cross ow,topography can have an eectUniv.Bualo 9Lee Wave,Separation,Coanda Eect...For plumes in a highspeed cross ow,topography can have an eectBut is this the case even when the plume is less dense than underlying layers?Univ.Bualo 9Over a range of Re,for the buoyant jet with ambient water density above plumedensity,there is reattachmentThe result is that particles that would otherwise settle through atmosphere aredeposited from a ground-hugging currentUniv.Bualo 10PrognosisUniv.Bualo 11PrognosisMost models do not provide full dynamic expression of separate phasesUniv.Bualo 11PrognosisMost models do not provide full dynamic expression of separate phasesIt is dicult to know what the turbulence intensity and spectrum inside a plume isUniv.Bualo 11PrognosisMost models do not provide full dynamic expression of separate phasesIt is dicult to know what the turbulence intensity and spectrum inside a plume isThere are unmodeled eectsUniv.Bualo 11PrognosisMost models do not provide full dynamic expression of separate phasesIt is dicult to know what the turbulence intensity and spectrum inside a plume isThere are unmodeled eectsUniv.Bualo 11Volcanic Plume ConvectionUniv.Bualo 12Volcanic Plume ConvectionUniv.Bualo 12